This invention relates to a ceramic heater suitable for use with an oxygen sensor adapted to sense the oxygen concentration of exhaust gases discharged from an internal combustion engine and a method of producing such a ceramic heater.
Normally, an oxygen sensor is located to monitor the oxygen content of the exhaust from an engine for air/fuel ratio feedback control. It is preferable to start an engine equipped with a turbo charger at an enriched air/fuel ratio. In this case, the temperature of the engine exhaust is about 280.degree. C. and is not enough for the oxygen sensor to operate in order. It is the current practice to heat the oxygen sensor to its activation temperature (about 350.degree. C.) with the use of a ceramic heater.
For example, Japanese Patent Kokai No. 63-146381 discloses a conventional oxygen sensor provided with a ceramic heater. The ceramic heater includes a ceramic heater core formed in a rod shape through an injection molding process. A heater pattern is printed on the outer peripheral surface of the heater core. The heater core is dipped along with the heater pattern in a ceramic slip and burnt to form a protective layer on the heater pattern. A conductive material is plated on the heater core outer peripheral surface not covered by the protective layer to form terminals for connection to a power source.
One of the difficulties encountered with such a conventional ceramic heater is the tendency toward defective contact since the terminals have curved surfaces. In addition, such a conventional ceramic heater has a great thermal capacity which requires a long time for the heater pattern to heat the oxygen sensor to its activation temperature. Furthermore, it is very difficult to uniform the thickness of the protective layer formed on the heater core on which the heater pattern has been printed without severe temperature and humidity control for the ceramic slip.